A wide variety of amines are used as curing agents for epoxy resins. Most of these amine curing agents contain primary amino groups. These amines are prone to react with carbon dioxide from the air forming carbamates (carbamatization) which represents an undesirable side reaction in such epoxy formulations. As a result, the cured epoxy formulation may have an aesthetically unsatisfactory surface often characterized by a waxy or oily surface layer, a white surface layer or spots of salt deposits occasionally accompanied by tackiness and incomplete surface curing. In some instances the marks are not visible but occur on contact with high humidity or water. These effects are generally known as “amine blushing” or “water spotting”.
This problem is even more severe when curing is carried out at low temperatures and high humidities since curing speed is reduced so that the available time for carbamatization side reactions by amines migrating to the surface is increased.
Polyamines having secondary amine groups could be used as curing agent since secondary amines are as much not subject to carbamatization but these polyamines are difficult to obtain and often very expensive. Another option is to use epoxy-amine adducts as curing agent, however these adducts usually have a high viscosity so that a dilution by solvents or benzyl alcohol is necessary to achieve an appropriate workability. As a result, according to the prior art, accelerators or diluents, for instance alcohols such as benzyl alcohol, are often added to the epoxy formulation comprising amine curing agents in order to avoid or reduce amine blushing.
Thus, commercial amine cured epoxy formulations are typically formulated with benzyl alcohol which is often used as a co-solvent in epoxy resins formulations in order to improve physical properties and appearance, in particular for low temperature curing applications. However, benzyl alcohol has its own disadvantages, since benzyl alcohol is a volatile organic compound (VOC) so that high VOC formulations are obtained, and large amounts of benzyl alcohol may have a negative impact on the mechanical properties of the cured epoxy resin.
In view of environmental and health concerns, low VOC epoxy formulations are advantageous so that it would be desirable to reduce or avoid benzyl alcohol in such formulations. In order to reduce the dependency on raw materials derived from petrochemical feedstock it is further desirable to have ingredients such as curing agents for use in epoxy formulations which can be derived from renewable resources.
Furan-based amines are such compounds derived from natural feedstock and have been previously described in the literature, e.g. A. Gandini, “Furans as offspring of sugars and polysaccharides and progenitors of a family of remarkable polymers: a review of recent progress”, Polym. Chem., 2010, 1, 245-251; and A. Gandini et al., Furans in Polymer Chemistry, Progr. Polym. Sci., 1997, 22, 1203.
Xiaodun He et. al., Journal of Polymer Science, Polymer Chemistry Edition, NY, US, vol. 30, no. 4, 30 Mar. 1992, pages 533-542, describe the evaluation of furfurylamines as curing agents for epoxy resins.